Method and Apparatus of Beam Training for MIMO Operation

ABSTRACT

The disclosed invention provides an efficient method for beam training to enable spatial multiplexing MIMO operation and spatial combining in a wireless network. The invention discloses a simple and efficient beam-training algorithm and protocol for MIMO operation that operates in high SNR condition for reliable MIMO operation. In one novel aspect, the best MIMO beam combinations are determined after TX sector sweeping and RX sector sweeping. In addition, the selection criteria includes not only signal quality, but also considers mutual interference and leakage among multiple MIMO spatial streams to improve overall MIMO performance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation, and claims priority under 35 U.S.C.§120 from nonprovisional U.S. patent application Ser. No. 13/899,540,entitled “Method and Apparatus of Beam Training for MIMO Operation,”filed on May 21, 2013, the subject matter of which is incorporatedherein by reference. Application Ser. No. 13/899,540, in turn, claimspriority under 35 U.S.C. §119 from U.S. Provisional Application No.61/650,220, entitled “Method and Apparatus for Beam Training for MIMOOperation,” filed on May 22, 2012, the subject matter of which isincorporated herein by reference.

TECHNICAL FIELD

The disclosed embodiments relate generally to wireless networkcommunications, and, more particularly, to beam training methods formultiple-input multiple-output (MIMO) operation.

BACKGROUND

Spatial multiplexing multiple input and multiple output (MIMO) techniqueis used to increase the data rate (and spectral efficiency) bytransmitting multiple data streams via different spatial pathssimultaneously. Spatial combining technique, on the other hand, refersto the technique that combines the same data stream via differentspatial paths to enhance signal quality. Spatial multiplexing andspatial combining techniques have been widely employed in mobilecommunications systems such as IEEE 802.11n (2.4 GHz and 5 GHz) and IEEE802.11ac (5 GHz). For 802.11n and 802.11ac, the signal wavelength islarge comparing to the feature size of objects in the propagationenvironment. As a result, NLOS signal propagation is dominated by thesignal scattering from various objects. Due to the severe scattering,OFDM signal is often used in such systems and the spatial multiplexingand spatial combining are done on a per-tone (per-subcarrier) basis inthe digital domain.

For higher frequency systems such as IEEE 802.11ad (60 GHz), the signalpropagation characteristics change as the signal wavelength becomessmall comparing to the feature size of objects in the propagationenvironment. As a result, signal propagation is dominated by ray-likepropagation with discrete paths in space. The signal quality can begreatly enhanced if either TX or RX antenna beams or both TX and RXantenna beams are directed toward strong spatial signal path. Theimproved signal quality via aligning the antenna beams with strongspatial signal path manifests both increased signal strength (highersignal-to-noise ratio) and reduced delay spread. Since the delay spreadis reduced, spatial combining can be wholly or partially done in RFdomain (instead of digital domain) to simplify implementation.

In general, phased-array antenna with steerable antenna beam in MIMOoperation provides antenna gain and enables mobility. Eigen-beamformingis one method of antenna beam training. The Eigen-beamforming requirestransmitter and receiver to estimate the channel response matrix first.The channel response matrix is then decomposed using singular valuedecomposition (SVD). The MIMO operation uses n dominant Eigen modes(corresponding to n spatial paths) for transmitting n spatial streams.The Eigen beamforming method suffers from the problem that the channelresponse matrix is obtained in lower signal-to-noise condition since nobeamforming is used during the channel estimation.

Another method of antenna beam training is multi-stage iterativetraining using power method. In the power method, the receiver sendsback the normalized receive vector in the n antennas to the transmitter.The transmitter uses the receive vector as the next transmit antennaweight. The antenna weight quickly converges to the first Eigen vectorafter a few iterations. This process continues until the n vectors(antenna weight vectors) are obtained. The power method suffers from theproblem that it only works (converges) in the presence of highsignal-to-noise ratio.

The beam training protocol provided in IEEE 802.11ad involves eithertransmitter or receiver to sweep through a number of antenna beamdirections to determine the beam with the best signal quality. Forefficient beam training, multiple stages of beam training are provided.The initial stage, called the SLS (sector level sweep), provides coarseantenna beam training. The subsequent stage, called the beam refinementprotocol or beam tracking, provides the fine-tuning of antenna beam forimproved pointing accuracy and higher signal quality. These beamtraining protocols are generally used to train a single spatial beam forthe transmission of a single data stream.

A solution is sought for training multiple antenna beam combinations toallow for multiple data streams for increased data rate, or to allowcombining of the same data stream for enhanced signal quality.

SUMMARY

The disclosed invention provides an efficient method for beam trainingto enable spatial multiplexing MIMO operation and spatial combining in awireless network. The invention discloses a simple and efficientbeam-training algorithm and protocol for MIMO operation that operates inhigh SNR condition for reliable MIMO operation without the drawbacks ofprior art methods.

In a first embodiment, an initiator and a responder exchangebeam-training parameters to start a MIMO training procedure. During TXsector sweeping, the initiator sends training packets through all TXsectors, while the responder receives the training packets withomni-direction beam. The responder sends back a set of selected TXsectors with good received signal quality. During RX sector sweeping,the initiator sends training packet with omni-direction beam, while theresponder receives the training packets through all RX sectors. Theresponder determines a set of selected RX sectors with good receivedsignal quality. During beam combination training, the initiator and theresponder sweep through the selected TX sectors and RX sectors together.The responder determines the best MIMO beam combinations for multipleMIMO spatial streams based on SNIR and sends back to the initiator.Finally, beam refinement is performed to fine-tune the antenna beams forimproved signal quality. In one novel aspect, the leakage from onespatial stream into the receive beam of another spatial stream isconsidered as interference for SNIR calculation.

In a second embodiment, an initiator and a responder exchangebeam-training parameters to start a MIMO training procedure. During TXsector sweeping, the initiator sends training packets through all TXsectors, while the responder receives the training packets withomni-direction beam. The responder sends back a set of selected TXsectors with good received signal quality. During RX sector sweeping,the initiator sends training packet using each selected TX sector, whilethe responder receives the training packets through all RX sectors. Theresponder selects one RX sector with good received signal quality foreach selected TX sector. The responder determines the best MIMO beamcombinations for multiple MIMO spatial streams based on SNIR and sendsback to the initiator. Finally, beam refinement is performed tofine-tune the antenna beams for improved signal quality. In one novelaspect, the leakage from one spatial stream into the receive beam ofanother spatial stream is considered as interference for SNIRcalculation.

Other embodiments and advantages are described in the detaileddescription below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a MU-MIMO operation with antenna beam training formultiple spatial streams in a wireless system in accordance with onenovel aspect.

FIG. 2 illustrates a simplified block diagram of a wireless device in awireless system in accordance with one novel aspect.

FIG. 3 illustrates a message/signal exchange flow of a first embodimentof antenna beam training for MIMO operation.

FIG. 4A illustrates a first step of the first embodiment of antenna beamtraining.

FIG. 4B illustrates a second step of the first embodiment of antennabeam training.

FIG. 4C illustrates a third step of the first embodiment of antenna beamtraining.

FIG. 5 illustrates a message/signal exchange flow of a second embodimentof antenna beam training for MIMO operation.

FIG. 6A illustrates a first step of the second embodiment of antennabeam training.

FIG. 6B illustrates a second step and a third step of the secondembodiment of antenna beam training.

FIG. 7 is a flow chart of a first embodiment of a method of antenna beamtraining for MIMO operation in accordance with a novel aspect.

FIG. 8 is a flow chart of a second embodiment of a method of antennabeam training for MIMO operation in accordance with a novel aspect.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 illustrates a MU-MIMO operation with antenna beam training formultiple spatial streams in a wireless system 100 in accordance with onenovel aspect. Wireless system 100 comprises an initiator 101 and aresponder 102. Both initiator 101 and responder 102 are equipped withantenna arrays to support MIMO operation for multiple spatial streams.To enable MIMO operation, initiator 101 signals to responder 102 tostart a MIMO training with a list of MIMO training parameters. Thepurpose of MIMO training is for antenna array beamforming, where bothtransmitting antennas and receiving antennas are steered with the bestbeam combinations to increase data rate and enhance signal quality.

In the example of FIG. 1, the intended direction is initiator 101 is thetransmitter of MIMO signal and responder 102 is the receiver of MIMOsignal. Note that an initiator can also initiate a MIMO training inwhich the initiator is the receiver of MIMO signal. In step 111,initiator 101 performs TX sector sweeping, where training packets aretransmitted to responder 102 through the TX sectors, each sectorcorresponds to a specific TX antenna beam/pattern (e.g.,direction/weight). During RX sector sweeping, training packets aretransmitted from initiator 101 to responder 102, which sweeps throughthe RX sectors, each sector corresponds to a specific RX antennabeam/pattern (e.g., direction/weight). In step 112, responder 102records the received signal quality (e.g., signal-to-noise ratio (SNR))and determines a number of beam combinations (selected TX and RX sectorpairs) based on the results of TX sector sweep and RX sector sweep. Thenumber of beam combinations needs to be greater or equal to the numberof spatial streams. In step 113, from the selected TX and RX sectorpairs, responder 102 determines the best MIMO beam combinations for themultiple MIMO spatial streams based on signal-to-(noise+interference)ratio (SNIR) criteria.

In one novel aspect, the simple and efficient beam training methodoperates in high SNR condition for reliable operation. Furthermore, byconsidering mutual interference or leakage among the multiple MIMOspatial streams, the MIMO beam combination selection is more accurate toimprove overall MIMO performance.

FIG. 2 illustrates a simplified block diagram of a wireless device 201in a wireless system in accordance with one novel aspect. Wirelessdevice 201 comprises memory 202, a processor 203, a scheduler 204, aMIMO encoder 205, a beamformer/precoder 206, a channel estimation module207, and a plurality of transceivers 211-214 coupled to a plurality ofantennas 215-218, respectively. The wireless device receives RF signalsfrom the antenna, converts them to baseband signals and sends them tothe processor. RF transceiver also converts received baseband signalsfrom the processor, converts them to RF signals, and sends out to theantenna. Processor 203 processes the received baseband signals andinvokes different functional modules to perform features in the device.Memory 202 stores program instructions and data to control theoperations of the device. FIG. 2 further illustrates functional modulesin the wireless device that carry out embodiments of the currentinvention. The functional modules may be implemented by hardware,firmware, software, or any combination thereof.

FIG. 3 illustrates a message/signal exchange flow of a first embodimentof antenna beam training for MIMO operation in a wireless communicationssystem 300. Wireless communications system 300 comprises an initiator301 and a responder 302. In step 310, the initiator sends a MIMObeam-training message to the responder to start a MIMO trainingprocedure. The beam-training message comprises MIMO training parameterssuch as the number of TX sectors, the number of RX sectors, the numberof MIMO spatial streams, the number of candidate beam combinations, andother relevant parameters. For example, the duration and timing oftraining packets may be included as part of the parameters. In theexample of FIG. 3, the initiator is the transmitter of MIMO signal.However, an initiator can also initiate a MIMO training in which it isthe receiver of MIMO signal.

In step 311, during TX sector sweeping, initiator 301 starts sendingtraining packets to responder 302. Each training packet is a shortpacket designed for beam training—allowing the receiver to measure thereceived signal quality, but not carrying extra data payload to reducetime. For TX sector sweeping, the training packets are sent through allthe TX sectors—one packet per sector with a gap (inter-frame spacing)between consecutive training packets. Responder 302 receives thetraining packets with an omni-direction antenna pattern and records thereceived signal quality for each TX sector. In step 312, responder 302feedbacks a set of selected TX sectors with good received signal qualityto initiator 301. In step 313, RX sector sweeping is performed.Initiator 301 transmits training packets with a semi-omni antennapattern while responder 302 sweep through all the RX sectors with dwelltime of each RX sector corresponding to the training packet duration andtiming. Responder 302 again records the received signal quality for eachRX sector. In step 314, responder 302 optionally feedbacks the candidatebeam combinations (e.g., a list of TX-RX sector pairs) to initiator 301.Based on the results of TX sector sweep and RX sector sweep, theselection of RX sectors is based on signal quality (e.g., SNR).

In step 315, the initiator and the responder start sweeping the selectedTX sectors and the selected RX sectors together for beam combinationtraining. During the beam combination training, the initiator transmitsa training packet through one of the selected TX sectors while theresponder receives the training packet through the paired RX sector inone beam combination. Because the initiator already knows the selectedTX-RX sector pairs, it knows how many times to send the training packetsfor each selected TX sector. In an alternative embodiment, the respondermight not feedback the beam combinations in step 314. As long as theinitiator knows the number of candidate beam combinations, it stillknows how many times to send the training packets for each selected TXsector for beam combination training. The responder records the signalquality for each selected TX-RX sector pair during the beam combinationtraining.

In step 316, responder 302 determines the best MIMO beam combinationsfor the multiple MIMO spatial streams. If there are two MIMO spatialstreams, then two best MIMO beam combinations are determined. Forspatial multiplexing, the best MIMO beam combinations are determinedbased on the highest SNIR. For spatial combining, the best MIMO beamcombinations are determined based on the highest total combined power(SUM power). In step 317, responder 302 feedbacks the best MIMO beamcombinations to initiator 301. Finally, in step 318, initiator 301 andresponder 302 perform beam refinement, which fine-tunes the antennabeams for improved pointing accuracy and higher signal quality. Moredetails of the first MIMO training embodiment are now described belowaccompanied with FIGS. 4A-4C.

FIG. 4A illustrates a first step of the first embodiment of antenna beamtraining. The first step involves TX sector sweeping after initializinga MIMO training between an initiator and a responder. In the example ofFIG. 4A, the initiator transmits training packets through totalthirty-two (32) TX sectors—sectors 1 to 32. The responder receives thetraining packets with an omni-direction beam. The responder then selectsfour TX sectors with the best signal quality (e.g., TX sectors 1, 9, 25,and 28). The responder also sends the selected TX sectors back to theinitiator.

FIG. 4B illustrates a second step of the first embodiment of antennabeam training. The second step involves RX sector sweeping. In theexample of FIG. 4B, the initiator transmits training packets using anomni-direction beam to the responder, while the responder receives thetraining packets sweeping through total sixteen (16) RX sectors—sectors1 to 16. The responder then selects four RX sectors with the best signalquality (e.g., RX sectors 3, 6, 8, and 15).

FIG. 4C illustrates a third step of the first embodiment of antenna beamtraining. The third step involves beam combination training using theselected TX sectors and the selected RX sectors. In the example of FIG.4C, the initiator transmits training packets sweeping through theselected TX sectors (1, 9, 25, and 28), while the responder receives thepackets sweeping through the selected RX sectors (3, 6, 8, and 15). Theresponder then records the signal quality (SNR) for all sixteen (4 TXsectors×4 RX sectors=16) beam combinations, as depicted by table 430.

The best MIMO beam combinations for multiple MIMO spatial streams areselected from the sixteen beam combinations. The best beam combinationtypically means the highest signal quality (SNR). However, in order toselect the best beam combinations for multiple spatial streams, theselection criteria needs to include the interference or leakage betweenthe spatial streams. Suppose TX1-RX3 sector pair and TX28-RX6 sectorpair are selected as the MIMO beam combinations for two MIMO spatialstreams SS1 and SS2, respectively. The received signal power from TX1 toRX6 becomes the interference to the TX28-RX6 pair, and the receivedsignal power from TX28 to RX3 becomes the interference to the TX1-RX3pair. After considering the mutual interference or leakage, theresponder determines the best two beam combinations for two spatialstreams.

FIG. 5 illustrates a message/signal exchange flow of a second embodimentof antenna beam training for MIMO operation in a wireless communicationssystem 500. Wireless communications system 500 comprises an initiator501 and a responder 502. In step 510, the initiator sends a MIMObeam-training message to the responder to start a MIMO trainingprocedure. The beam-training message comprises MIMO training parameterssuch as the number of TX sectors, the number of RX sectors, the numberof MIMO spatial streams, the number of candidate beam combinations, andother relevant parameters. For example, the duration and timing oftraining packets may be included as part of the parameters. In theexample of FIG. 5, the initiator is the transmitter of MIMO signal.However, an initiator can also initiate a MIMO training in which it isthe receiver of MIMO signal.

In step 511, during TX sector sweeping, initiator 501 starts sendingtraining packets to responder 502. Each training packet is a shortpacket designed for beam training—allowing the receiver to measure thereceived signal quality, but not carrying extra data payload to reducetime. For TX sector sweeping, the training packets are sent through allthe TX sectors—one packet per sector with a gap (inter-frame spacing)between consecutive training packets. Responder 502 receives thetraining packets with omni-direction antenna pattern and records thereceived signal quality for each TX sector. In step 512, responder 502feedbacks a set of selected TX sectors with good received signal qualityto initiator 501. In step 513, RX sector sweeping is performed. In thesecond embodiment, instead of using omni-direction beam for transmittingtraining packets, initiator 501 transmits through one of the selected TXsectors a number of training packets (corresponding to the number of RXsectors to be swept), while responder 502 sweeps through all the RXsectors. The initiator repeats this process for each selected TX sectorwhile the responder sweeps through all the RX sectors for each selectedTX sector. The dwell time of each RX sector corresponds to a trainingpacket duration and timing. Responder 502 again records the receivedsignal quality and selects one RX sector with good signal quality foreach selected TX sector.

In step 514, responder 502 determines the best MIMO beam combinationsfor multiple MIMO spatial streams based on the results of the TX sectorsweeping and the RX sector sweeping. No additional beam combinationtraining is necessary in the second embodiment. If there are two MIMOspatial streams, then the two best MIMO beam combinations aredetermined. For spatial multiplexing, the best MIMO beam combinationsare determined based on the highest SNIR. For spatial combining, thebest MIMO beam combinations are determined based on the highest totalcombined power (SUM power). In step 515, responder 502 feedbacks thebest MIMO beam combinations to initiator 501. Finally, in step 516,initiator 501 and responder 502 perform beam combination refining, whichfine-tunes the antenna beams for improved pointing accuracy and highersignal quality. More details of the second MIMO training embodiment arenow described below accompanied with FIGS. 6A-6B.

FIG. 6A illustrates a first step of the second embodiment of antennabeam training. The first step involves TX sector sweeping afterinitializing a MIMO training between an initiator and a responder. Inthe example of FIG. 6A, which is similar to FIG. 4A of the firstembodiment, the initiator transmits training packets through totalthirty-two (32) TX sectors—sectors 1 to 32. The responder receives thetraining packets with an omni-direction beam. The responder then selectsfour TX sectors with the best signal quality (e.g., TX sectors 1, 9, 25,and 28). The responder also sends the selected TX sectors back to theinitiator.

FIG. 6B illustrates a second step and a third step of the secondembodiment of antenna beam training. The second step involves RX sectorsweeping. In the example of FIG. 6B, the initiator transmits trainingpackets using each of the selected TX sector (1, 9, 25, and 28) to theresponder, while the responder receives the training packets sweepingthrough total sixteen (16) RX sectors—sectors 1 to 16. The responderthen selects one RX sector with the highest signal quality for each ofthe selected TX sector (e.g., RX sectors 3, 6, 8, and 15 for TX sectors1, 9, 25, and 28 respectively).

The third step involves the final selection of the best MIMO beamcombinations. The already selected four TX-RX sector pairs are based onsignal quality. In order to find the best beam combinations for multiplespatial streams, the selection criteria needs to include the mutualinterference or leakage between the spatial streams. After consideringthe mutual interference or leakage, the best two beam combinations arefinally determined. For example, TX1-RX3 sector pair is selected for afirst spatial stream SS#1, and TX28-RX6 sector pair is selected for asecond spatial stream SS#2. In this case, the received signal power fromTX1 to RX6 becomes the interference to the TX28-RX6 pair, and thereceived signal power from TX28 to RX3 becomes the interference to theTX1-RX3 pair. Based on both the SNR and SNIR information, the TX1-RX3and TX28-RX6 beam combinations are the best beam combinations for MIMOSS#1 and SS#2.

Note that the second embodiment overall requires more training packetsas compared to the first embodiment. From implementation perspective,the difference between the first embodiment and the second embodiment iswhether the transmitting device needs to send training packets usingomni-direction antenna pattern. The second embodiment does not requirethe implementation of semi-omni transmit antenna. In general, receiveromni-pattern is easier to form since one single antenna element wouldprovide near omni pattern. The receiver omni-antenna gain is lower thanthe array antenna gain by the array gain. For transmitter, however, itis difficult to provide an omni-pattern. If a single antenna is used forgenerating omni-directional pattern, not only the antenna gain isreduced by the array gain but also the power gain is reducedproportional to how many power amplifiers are not used. As a result, theeffective isotropic radiated power (EIRP) is reduced by array gain pluspower gain.

FIG. 7 is a flow chart of a first embodiment of a method of antenna beamtraining for MIMO operation in accordance with a novel aspect. In step701, an initiator communicates with a responder a MIMO beam-trainingmessage to start a MIMO training procedure in a wireless network. Theinitiator is the transmitter of MIMO signal and the responder is thereceiver of MIMO signal. Alternatively, the receiver of MIMO signal mayalso initiate the MIMO training procedure. In step 702, TX sectorsweeping is started. The initiator sends training packets through all TXsectors, and the responder receives the training packets withomni-direction beam. In step 703, the responder sends a set of selectedTX sectors with good received signal quality back to the initiator. Instep 704, RX sector sweeping is started. The initiator sends trainingpackets with omni-direction beam, and the responder receives thetraining packets through all RX sectors. The responder then determines aset of selected RX sectors with good received signal quality. In step705, beam combination training is started. The initiator and theresponder sweep through the selected TX sectors and RX sectors together.The responder records the received signal quality. In step 706, theresponder determines the best MIMO beam combinations for multiple MIMOspatial streams based on signal quality and based on interference orleakage among the different spatial streams. In step 707, beamrefinement is performed to fine-tune the antenna beams for improvedsignal quality.

FIG. 8 is a flow chart of a second embodiment of a method of antennabeam training for MIMO operation in accordance with a novel aspect. Instep 801, an initiator communicates with a responder a MIMObeam-training message to start a MIMO training procedure in a wirelessnetwork. The initiator is the transmitter of MIMO signal and theresponder is the receiver of MIMO signal. Alternatively, the receiver ofMIMO signal may also initiate the MIMO training procedure. In step 802,TX sector sweeping is started. The initiator sends training packetsthrough all TX sectors, and the responder receives the training packetswith omni-direction beam. In step 803, the responder sends a set ofselected TX sectors with good received signal quality back to theinitiator. In step 804, RX sector sweeping is started. The initiatorsends training packets using one of the selected TX sectors, and theresponder receives the training packets through all RX sectors. Theinitiator repeats the process for each selected TX sector. The responderthen determines a set of selected RX sectors with good received signalquality for each selected TX sector. In step 805, the responderdetermines the best MIMO beam combinations for multiple MIMO spatialstreams based on signal quality and based on interference or leakageamong the different spatial streams. In step 806, beam refinement isperformed to fine-tune the antenna beams for improved signal quality.

Although the present invention has been described in connection withcertain specific embodiments for instructional purposes, the presentinvention is not limited thereto. Accordingly, various modifications,adaptations, and combinations of various features of the describedembodiments can be practiced without departing from the scope of theinvention as set forth in the claims.

What is claimed is:
 1. An initiator device, comprising: a transceiverthat communicates a beam-training message that initiates a multipleinput and multiple output (MIMO) training procedure in a wirelessnetwork; a transmitter that transmits training packets using all TXsectors during TX sector sweeping and in response obtaining a set ofselected TX sectors, wherein each of the TX sectors corresponds to aspecific TX antenna beam, wherein the transmitter also transmitstraining packets using omni-direction antenna pattern during RX sectorsweeping and in response obtaining a set of selected RX sectors, andwherein each of the RX sectors corresponds to a specific RX antennabeam; a processor that starts MIMO beam combination training based onthe selected TX and RX sectors by transmitting training packets usingthe selected TX sectors, wherein each MIMO beam combination includes oneof the selected TX sectors and one of the selected RX sectors; and aMIMO encoder that obtains one or more best MIMO beam combinations formultiple MIMO spatial streams, wherein each of the best MIMO beamcombinations is selected based on the results of the MIMO beamcombination training.
 2. The initiator of claim 1, wherein thebeam-training message comprises parameters including a number of TXsectors, a number of RX sectors, a number of MIMO spatial streams, and anumber of beam combinations.
 3. The initiator of claim 1, wherein theselected TX and RX sectors are determined based on signal to noiseratios (SNRs).
 4. The initiator of claim 1, wherein the best MIMO beamcombinations are determined based on signal to noise plus interferenceratios (SNIRs).
 5. The initiator of claim 1, wherein the beamcombination training involves sweeping the selected TX sectors and theselected RX sectors together.
 6. A responder device, comprising: atransceiver that communicates a beam-training message that initiates amultiple input and multiple output (MIMO) training procedure in awireless network; a receiver that receives training packets usingomni-direction antenna pattern during TX sector sweeping and in responsedetermine a set of selected TX sectors, wherein each of the TX sectorscorresponds to a specific TX antenna beam, wherein the receiver alsoreceives training packets using all RX sectors during RX sector sweepingand in response determine a set of selected RX sectors, wherein each ofthe RX sectors corresponds to a specific RX antenna beam; a processorthat starts MIMO beam combination training based on the selected TX andRX sectors by receiving training packets from the selected TX sectorsusing the selected RX sectors, wherein each of the MIMO beamcombinations includes one of the selected TX sectors and one of theselected RX sectors; and a MIMO encoder that determines one or more bestMIMO beam combinations for multiple MIMO spatial streams based on theresults of the MIMO beam combination training.
 7. The responder of claim6, wherein the beam-training message comprises parameters including anumber of TX sectors, a number of RX sectors, a number of MIMO spatialstreams, and a number of beam combinations.
 8. The responder of claim 6,wherein the selected TX and RX sectors are determined based on signal tonoise ratios (SNRs).
 9. The responder of claim 6, wherein the best MIMObeam combinations are determined based on signal to noise plusinterference ratios (SNIRs).
 10. The responder of claim 6, wherein thetraining packets during the RX sector sweeping are transmitted with anomni-direction antenna pattern.
 11. An initiator device, comprising: atransceiver that communicates a beam-training message that initiates amultiple input and multiple output (MIMO) training procedure in awireless network; a transmitter that transmits training packets usingall TX sectors during TX sector sweeping and in response obtaining a setof selected TX sectors, wherein each of the TX sectors corresponds to aspecific TX antenna beam; wherein the transmitter also transmitstraining packets using the selected TX sectors during RX sector sweepingand in response obtaining a set of selected RX sectors corresponding toeach of the selected TX sectors, wherein each of the RX sectorscorresponds to a specific RX antenna beam; and a MIMO encoder thatobtains one or more best MIMO beam combinations for multiple MIMOspatial streams, wherein each of the MIMO beam combinations includes oneof the selected TX sectors and one of the selected RX sectors, whereinthe selected TX and RX sectors are determined based on signal to noiseratios (SNRs) and the selected MINO beam combinations are determinedbased on signal noise plus interference ratios (SNIRs).
 12. Theinitiator of claim 11, wherein the beam-training message comprisesparameters including a number of TX sectors, a number of RX sectors, anumber of MIMO spatial streams, and a number of beam combinations. 13.The initiator of claim 11, wherein the training packets during the RXsector sweeping are repeated for a number of total RX sectors for eachof the selected TX sectors.
 14. A responder device, comprising: atransceiver that communicates a beam-training message that initiates amultiple input and multiple output (MIMO) training procedure in awireless network; a receiver that receives training packets usingomni-direction antenna pattern during TX sector sweeping and in responsedetermine a set of selected TX sectors, wherein each TX sectorcorresponds to a specific TX antenna beam, wherein the receiver alsoreceives training packets using all RX sectors during RX sector sweepingand in response determining a set of selected RX sectors correspondingto each selected TX sector, wherein each RX sector corresponds to aspecific RX antenna beam; and a MIMO encoder that determines one or morebest MIMO beam combinations from the selected TX and RX sectors formultiple MIMO spatial streams, wherein each MIMO beam combinationincludes one of the selected TX sectors and one of the selected RXsectors, wherein the selected TX and RX sectors are determined based onsignal to noise ratios (SNRs) and the selected MINO beam combinationsare determined based on signal noise plus interference ratios (SNIRs).15. The responder of claim 14, wherein the beam-training messagecomprises parameters including a number of TX sectors, a number of RXsectors, a number of MIMO spatial streams, and a number of beamcombinations.
 16. The responder of claim 14, wherein the trainingpackets during the RX sector sweeping are transmitted using each of theselected TX sectors.